1. Field of the Invention
The present invention relates to a power transfer device adapted for use in an automotive vehicle.
2. Discussion of the Prior Art
Disclosed in Japanese Patent Laid-open Publication 2000-234635 is a power transfer device of this kind which comprises a main clutch, a cam mechanism and an electromagnetic pilot clutch mechanism coaxially assembled within a cylindrical space between an external cylindrical rotary member and an internal rotary member. In the power transfer device, an electromagnetic coil of the pilot clutch mechanism is applied with an electric current to produce a pilot torque, the cam mechanism is applied with the pilot torque to produce thrust force in an axial direction, and the main clutch is engaged by the thrust force applied from the cam mechanism to effect transmission of a driving torque between the rotary members.
In use of the power transfer device in a four wheel drive vehicle, harmful noises will occur if differential rotation of the rotary members is reversed in a condition where the main clutch is being engaged by the thrust force applied from the cam mechanism. For example, in use of the power transfer device in a four wheel drive vehicle of the rear-wheel drive type, the differential rotation of the rotary members is reversed if the rotation speed of front road wheels becomes higher than that of rear road wheels due to turning of the vehicle at start. In such an instance, there will occur harmful noises due to clearances at the splined portions of the component parts, defacement of the component parts or a cam angle of the cam mechanism. Such a phenomenon will occur when the vehicle is steered to a straight travel from turning. In use of the power transfer device in a four wheel drive vehicle of the front-wheel drive type or the rear-wheel drive type, the differential rotation of the rotary members is reversed when the vehicle is driven backward from a forward travel or vice versa.
It is, therefore, a primary object of the present invention to provide a power transfer device capable of eliminating the occurrence of harmful noises when differential rotation of the rotary members is reversed in the foregoing conditions.
According to the present invention, the object is accomplished by providing a power transfer device including a main clutch, a cam mechanism and an electromagnetic pilot clutch mechanism coaxially assembled within a cylindrical space between an external cylindrical rotary member and an internal rotary member, wherein an electromagnetic coil of the pilot clutch mechanism is applied with an electric current to produce a pilot torque, the cam mechanism is applied with the pilot torque to produce thrust force in an axial direction and the main clutch is engaged by the thrust force applied from the cam mechanism to effect drive connection between the rotary members wherein the power transfer device includes means for decreasing a residual thrust force of the cam mechanism at an instance when differential rotation of the rotary members is reversed.
In a practical embodiment of the present invention, the power transfer device is provided with an electric control apparatus for decreasing the electric current applied to the electromagnetic coil of the pilot clutch mechanism at an instance when the differential rotation of the rotary members is reversed.
In another practical embodiment of the present invention, the power transfer device is provided with mechanical means for determining a clearance at a portion of the internal rotary member splined to a cam member of the cam mechanism smaller than a clearance at a portion of the internal rotary member splined to a set of clutch plates of the main clutch.
In a further practical embodiment of the present invention, the power transfer device is provided with mechanical means for determining a cam angle of the cam mechanism larger than a rotation angle caused by a clearance at a portion of a cam member of the cam mechanism splined to a set of clutch plates of the pilot clutch mechanism.